In 1997 Stefan Mittmann is being elected in the Board of Directors of the Hellenic Purchasing Institute and assumes the position of General Secretary. He holds this position after several reelections until 2013. The Hellenic Purchasing Institute (HPI) is non-profit organization, dedicated to the advancement of the procurement and supply management profession. It is the only Greek body member of the International Federation of Purchasing and Supply Management (IFPSM). HPI is organizing professional events and operates a long term educational programme for purchasing professionals in Greece.

In 2009 Stefan Mittmann is being elected in the Board of Directors of the Hellenic Association of Photovoltaic Companies (HELAPCO) and assumes the position of the Treasurer. He is being reelected for a second term in 2011. He represents HELAPCO in a number of events and puts his efforts in developing a long term, well structured and sustainable PV market.

As an alumni and active member of the German School of Athens educational institution, Stefan Mittmann is being elected as financial auditor for the years 2005 - 2010.

In 2013 he is being elected into the Board of Directors of the Institution for a term of 3 years. He participates in the school´s promotion and personnel task groups. He is especially interested in advancing the Greek-German relationships between the Greek and German communities within a joint European development.